The invention relates to a display device comprising an illumination system having a radiation source for supplying a radiation beam, and at least a diffusing display panel comprising an optically active diffusing medium which is switchable between a transparent, polarization-maintaining state and a diffusing, depolarizing state for the radiation beam, and a first polarizer having a first polarization effect and a second polarizer having a second, complementary polarization effect, said polarizers enclosing the medium.
The invention also relates to a diffusing display panel.
The display devices under consideration may be grouped into two types, viz. image projection devices and flat-panel display devices.
An image projection device is a device in which an image generated by means of a display panel, for example a diffusing panel, is imaged at a relatively large distance and in a magnified form on a projection screen by means of a projection lens system. The illumination system for this device comprises, for example a radiation source and a beam-concentrating optical system.
In a flat-panel display device an image is also generated by a display panel. The dimension of this device in the direction transverse to the display panel is relatively small, much smaller than the dimension of an image projection device. In a flat-panel display device a viewer directly watches the display panel so that this device may also be referred to as a direct-vision device. The illumination system used may be, for example the assembly of a radiation source and an optically transparent plate of, for example PMMA used as a waveguide, while the radiation source is formed as a peripheral illumination system for this waveguide, as described, for example in U.S. Pat. No. 4,985,809. The illumination system may alternatively be a known blacklight illumination system.
The display device may be, for example a video display device or a monitor of a computer system, or the display device for an instrument panel.
In a display device in which the display panel comprises a TN LCD panel, an acceptable contrast can be achieved only within a limited viewing angle. The viewing angle may be increased by making use of diffusing display panels. Since the increase of the viewing angle will be at the expense of the contrast, the diffusing display panel is arranged between two absorbing polarizers having complementary states of polarization. Such display panels are known per se from, for example the article: "A full-colour TFT-LCD with a polymer-dispersed structure" by H. Yoshida et al. in Japan Display '92, pp. 631-634. The display panel described in this article has a PDLC layer (Polymer-Dispersed Liquid Crystal) as an optically active diffusing medium which is enclosed between two absorbing polarizers having complementary directions of polarization. The PDLC layer consists of a liquid crystalline material which is dispersed in the form of drops in a transparent polymer material. If no voltage is applied across the layer, the molecules within the drops have a given net orientation, but the orientation between the drops themselves is different. The radiation from a radiation source is polarized by the first polarizer. By diffusion within the PDLC, the polarized radiation is depolarized and consequently substantially 50% is passed by the second polarizer towards the viewer. The display panel is in the bright state in this case. However, if a voltage is applied, in other words if picture information to be displayed is applied to the display panel, and the molecules are thus subjected to an electric field, the molecules within the drops will orient substantially perpendicularly to the substrate and the PDLC itself becomes transparent. The radiation from the radiation source is again polarized by the first polarizer but passed unobstructed through the PDLC and consequently absorbed by the second polarizer. In this case the display panel displays a dark picture.
The PDLC within a pixel is thus switchable between a diffusing and a transparent state, which corresponds to a light-transmissive and an opaque state, respectively, of the relevant pixel of the display panel.
A drawback is, however, that only 25% of the light supplied by the radiation source can be used for the formation of a picture, even when ideal absorbing polarizers are used, because a loss of substantially 50% occurs in both polarizers due to absorption of the unsuitable direction of polarization. There should be sufficient light for the viewer, both in a direct-vision device and in an image projection device.
To compensate for the loss of light in the polarizers, high-power lamps must be used in both devices, which lamps consume much electric power, have a shorter lifetime and may have to be cooled.
If a portable direct-vision display device is used, the batteries will have a shorter lifetime because they must feed a high-power lamp. Moreover, a higher intensity will lead to hotter polarizers.
If an image projection device is used, the device should be provided with a radiation source having a high intensity.
Since the light intensity incident on the polarization means is high in both cases, the polarization means are considerably heated due to the absorption. Since these polarizers are present proximate to the display panel in the devices under consideration, it may be necessary to incorporate a cooling system, which renders the display device more complicated and more expensive and produces a troublesome noise.